unlines package:io-streams

unlines :: OutputStream ByteString -> IO (OutputStream ByteString)
io-streams System.IO.Streams.ByteString
Intersperses string chunks sent to the given OutputStream with newlines. See intersperse and unlines. 
ghci> os <- Streams.unlines Streams.stdout
ghci> Streams.write (Just "Hello,") os
Hello
ghci> Streams.write Nothing os
ghci> Streams.write (Just "world!") os
world!